In radio communication systems that are widely used at present, a transmitting apparatus (such as a radio base station) sends data to a receiving apparatus (such as a mobile station) by radio. The transmitting apparatus performs radio communication by using part or all of a predetermined frequency band (system band) in the radio communication system. The receiving apparatus extracts data from a received signal by generating internally an oscillator signal synchronized with the signal received from the transmitting apparatus.
The frequency of the radio signal received by the receiving apparatus may depend on the remote transmitting apparatus or may vary with the state of the radio propagation path. Accordingly, the receiving apparatus can perform automatic frequency control (AFC) to adjust the frequency of the internal oscillator signal. In AFC, any difference in frequency (frequency offset) is detected through a comparison between the frequency of the received signal and the frequency of the reference signal (feedback signal of the oscillator signal, for instance), and the oscillation frequency is adjusted in accordance with the detected frequency offset.
An oscillator signal of a desired frequency can be obtained by using a voltage controlled oscillator (VCO). In this method, the receiving apparatus converts the detected frequency offset to a voltage and applies the obtained voltage to the VCO, thereby adjusting the oscillation frequency of the VCO.
The receiving apparatus can adjust the phase while adjusting the frequency. For the phase adjustment, a phase locked loop (PLL) circuit is used, for example. A known technology reduces the time needed to adjust the frequency or phase. To be more specific, the technology conducts phase adjustment in two stages: Open loop and closed loop (see Japanese Laid-open Patent Publication No. 2002-118460, for instance).
If the frequency offset is detected by digital processing, conversion from the detected frequency offset to voltage accompanies digital-to-analog conversion. In one possible method, the detected amount of frequency offset (a value within the range of −10 to +10 parts per million (ppm), for instance) is converted to a code having a predetermined number of bits (an eight-bit code that can represent “0” to “255,” for instance), and the code is converted to voltage by a digital-to-analog converter (DAC).
Generally available DACs, however, have a fixed relationship between input code and output voltage as predetermined in accordance with the design specifications. Accordingly, in the conventional conversion method, the resolution of digital-to-analog conversion could cause a bottleneck, making it impossible to raise the precision of oscillation frequency adjustment sufficiently. The amount of change in voltage output from the DAC corresponding to an increment of “1” in input code determines the limit of precision of oscillation frequency adjustment.